1. Field of the Invention
The present invention relates to a vertical sync signal generator in a device such as a digital video decoder that decodes video signals. This vertical sync signal generator can generate vertical sync signals with no delay even if, for example, the inputted video signals temporarily enter the state of a weakened electric field.
2. Description of the Related Art
FIG. 8 is a general waveform diagram showing a vertical sync pulse in a vertical blanking interval in a regular television signal (hereafter “TV signal”).
Synthesized video signals are generally referred to as TV signals, and these include video signals for reproducing images and sync signals for assembling images. For synchronization signals, there are horizontal sync signals VS and vertical sync signals HS, and these are respectively placed in TV signal waveforms inside the horizontal blanking intervals and the vertical blanking intervals.
In the vertical blanking interval T shown in FIG. 8, there is a vertical sync pulse interval T2 of three lines made from vertical sync signals HS. The signals of this portion have cuts that are made in positive pulses P2 (cut pulses) with narrow widths repeated at a frequency of two times that of the horizontal sync frequency. An equivalent pulse interval T1 of three lines and an equivalent pulse interval T3 of three lines are provided directly before and after the vertical sync pulse interval T2. At each equivalent pulse interval T1, T3, six narrow and thin equivalent pulses P1 continue at the same frequency as the cut pulses P2. The equivalent pulses P1 and the cut pulses P2 inside the vertical blanking interval T fulfill the role of stably maintaining the horizontal sync on the receiver side.
A horizontal sync signal HS is generated by detecting a vertical sync pulse HSP. An example of the configuration of a conventional vertical sync signal generator is shown in FIG. 9.
FIG. 9 is a general structural block diagram showing an example of the configuration of a conventional vertical sync signal generator, and FIG. 10 is a time chart showing the operation of the vertical sync signal generator of FIG. 9.
With this vertical sync signal generator, a method is used where a vertical sync signal HS is generated with a vertical sync signal generating circuit 5 via a logical sum gate 4 (hereafter, “OR gate”) with a detection signal S1, only when a vertical sync pulse HSP from an inputted video signal PS has been detected with the vertical sync pulse detection circuit 1. Or, a method is used where, when the vertical sync pulse detection circuit 1 has detected a vertical sync pulse HSP from the inputted video signal PS, counting is performed with an upright frame cycle counter 2 where the maximum count value (MAX) is 262-263 while generating a vertical sync signal HS from the detection signal S1 with the vertical sync signal generating circuit 5 via the OR gate 4. This count value S2 is decoded with a timing generating decoder circuit 3, and a timing signal S3 of the position where the next vertical sync pulse HSP should come is sought; A vertical sync signal HS is generated at this position as well with the vertical sync signal generating circuit 5 via the OR gate 4, and this signal is synchronized with the line phase and outputted.
Conventional patent publications relating to this type of vertical sync signal generator include, for example, the Official Gazette of JP-A (Laid-open) No. 10-117296.
Nonetheless, there are certain problems with conventional vertical sync signal generators. When the vertical sync pulse detection circuit 1 makes a mistake and detects a vertical sync pulse HSP due to the noise included in the video signal PS when there is a weak electric field, this detection signal S1 is imparted to the vertical sync signal generating circuit 5 via the OR gate 4. Accordingly, there has been a problem in that erroneous vertical sync signals HS are generated by this the vertical sync signal generating circuit 5. And noise at the time of a weak electric field is generated more easily with TV signals than when compared to video signals.